Hollow structure and method for producing hollow structure

Abstract

To provide a hollow structure with superior moldability and superior restorability, and a method for producing the hollow structure.SOLUTION: A hollow structure 10 according to the present invention has a core layer 20 composed of a sheet material having a predetermined irregular shape formed by vacuum molding, where a plurality of cells S are arranged side by side. The sheet material is composed of a resin material containing polypropylene resin and thermoplastic elastomer. The sheet material has a flexural modulus of 200 MPa or more.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a hollow structure and a method for manufacturing a hollow structure. [Background technology]

[0002] Since hollow structures are lightweight yet have appropriate strength, they may be used as components of various vehicles, building materials, etc. Patent Document 1 describes an invention relating to a hollow structure that includes a core layer in which a plurality of cells are arranged side by side, and skin layers bonded to the upper and lower surfaces of the core layer. Such a hollow structure is composed of a sheet material having an uneven shape obtained by plastically deforming a polypropylene resin sheet by vacuum forming. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2022-105542 Summary of the Invention [Problem to be solved by the invention]

[0004] However, a hollow structure made of a polypropylene resin sheet has low restoring property against external pressure, and therefore a hollow structure with improved restoring property is required. In addition, since the hollow structure is made of a sheet material having a predetermined uneven shape formed by vacuum molding, it is also important that the moldability is not affected. [Means for solving the problem]

[0005] In order to solve the above problems, the hollow structure of the present invention is made of a sheet material having a predetermined uneven shape formed by vacuum molding, and has a core layer in which a plurality of cells are arranged side by side. The sheet material is made of a resin material containing a polypropylene resin and a thermoplastic elastomer, and the bending elastic modulus of the sheet material is 200 MPa or more.

[0006] According to the above-mentioned configuration, the resilience against external pressure can be improved compared to a hollow structure made of a sheet material made of a synthetic resin such as polypropylene resin. Since the flexural modulus of the sheet material is set to 200 MPa or more, the moldability is not affected when a sheet material having a predetermined uneven shape is obtained by vacuum molding.

[0007] In the above structure, the content of the thermoplastic elastomer in the sheet material is preferably 10% by mass or more. According to the above-mentioned configuration, the restoring property against external pressure can be improved as compared with a hollow structure constituted by a sheet material made of synthetic resin such as polypropylene resin.

[0008] In the above configuration, the polypropylene resin is preferably a block copolymer. According to the above configuration, when a sheet material having a predetermined concave and convex shape is obtained by vacuum forming, the formability can be further improved.

[0009] In the above configuration, the sheet material having the concave and convex shape is folded to provide a plurality of cells in parallel, and the bending elastic modulus of the sheet material is preferably 300 MPa or more.

[0010] According to the above configuration, even when the sheet material having the predetermined concave and convex shape formed by vacuum molding is folded to arrange a plurality of cells side by side, there is no effect on manufacturing characteristics. In order to solve the above problems, the manufacturing method of the hollow structural body of the present invention includes a forming step in which the sheet material is vacuum molded to form a predetermined uneven shape.

[0011] According to the above-mentioned configuration, when a sheet material having a predetermined concave and convex shape is obtained by vacuum forming, the formability is not affected, and therefore a hollow structure having excellent restorability against external pressure can be efficiently manufactured.

[0012] In the above-mentioned configuration, it is preferable that the molding step is followed by a folding step in which the sheet material is folded to arrange a plurality of cells side by side, and the bending elastic modulus of the sheet material is 300 MPa or more.

[0013] According to the above-mentioned configuration, even when the sheet material having a predetermined concave and convex shape formed by vacuum molding is folded to arrange a plurality of cells side by side, the manufacturing characteristics are not affected, and therefore, a hollow structure having excellent resilience against external pressure can be efficiently manufactured. Effect of the Invention

[0014] According to the present invention, a hollow structure having excellent moldability and excellent restoring property can be obtained. [Brief description of the drawings]

[0015] [Figure 1] Fig. 1(a) is a perspective view of the hollow structure of this embodiment, Fig. 1(b) is a cross-sectional view taken along line 1b-1b in Fig. 1(a), and Fig. 1(c) is a cross-sectional view taken along line 1c-1c in Fig. 1(a). [Diagram 2] Fig. 2(a) is a perspective view of the sheet material constituting the core layer, Fig. 2(b) is a perspective view showing the sheet material in the middle of being folded, and Fig. 2(c) is a perspective view showing the sheet material in the folded state. [Diagram 3] Fig. 3(a) is a perspective view of a sheet material constituting a core layer of a modified example, Fig. 3(b) is a perspective view showing the sheet material in the middle of being folded, and Fig. 3(c) is a perspective view showing the sheet material in the folded state. [Figure 4] Fig. 4(a) is a perspective view of a hollow structure of a modified example, Fig. 4(b) is a plan view of a sheet material of the hollow structure of the modified example, and Fig. 4(c) is a cross-sectional view taken along line 4c-4c of Fig. 4(b). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] An embodiment of the present invention will now be described with reference to FIG. As shown in Figures 1(a) to 1(c), the hollow structure 10 of this embodiment comprises a core layer 20 having a plurality of cells S arranged side by side therein, a skin layer 30 joined to the upper surface of the core layer 20, and a skin layer 40 joined to the lower surface of the core layer 20.

[0017] (Resin material) The core layer 20 and the skin layers 30, 40 are formed from sheets made of a resin material including polypropylene resin and a thermoplastic elastomer, such as polystyrene-based elastomers, olefin-based elastomers, polyester-based elastomers, polyvinyl chloride-based elastomers, polyurethane-based elastomers, and polyamide-based elastomers.

[0018] Further, specific examples of polystyrene-based elastomers include styrene-butadiene block copolymer (SBR), hydrogenated styrene-butadiene block copolymer (SEB), styrene-butadiene-styrene block copolymer (SBS), styrene-butadiene-butylene-styrene block copolymer (SBBS), styrene-ethylene-butylene-styrene block copolymer (SEBS), styrene-isoprene block copolymer (SIR), styrene-ethylene-propylene block copolymer (SEP), styrene-isoprene-styrene block copolymer (SIS), styrene-ethylene-propylene-styrene block copolymer (SEPS), styrene-ethylene-ethylene-propylene-styrene block copolymer (SEEPS), and the like.

[0019] Furthermore, specific examples of olefin-based elastomers include, for example, ethylene-propylene copolymers, ethylene-α-olefin copolymers, propylene-α-olefin copolymers, butene-α-olefin copolymers, ethylene-propylene-α-olefin copolymers, ethylene-butene-α-olefin copolymers, propylene-butene-α-olefin copolymers, ethylene-propylene-butene-α-olefin copolymers, and the like.

[0020] These thermoplastic elastomers may be used alone or in combination of two or more. Among these specific examples, polystyrene-based elastomers are preferred from the viewpoint of excellent restoring ability against external pressure of the hollow structure 10 without affecting moldability.

[0021] The compression set of the thermoplastic elastomer is not particularly limited, but is preferably 50% or less, and more preferably 40% or less. By making the compression set 50% or less, the restoring ability of the hollow structure 10 against external pressure can be further improved.

[0022] The compression set of the thermoplastic elastomer can be measured by a test method in accordance with JIS K 6262:2013. Examples of polypropylene resins include homopolymers, which are single polymers of propylene alone, random copolymers containing a specific ratio of ethylene as a comonomer, block copolymers, etc. Among these, block copolymers are preferred from the viewpoint of improving physical properties and also improving moldability when a sheet material having a specific uneven shape is obtained by vacuum molding.

[0023] The lower limit of the content of the thermoplastic elastomer in the sheet material is set appropriately, but is preferably 10% by mass or more, more preferably 15% by mass or more, and further preferably 20% by mass or more. When the content is 10% by mass or more, the restoring property of the hollow structure 10 against external pressure can be further improved.

[0024] The upper limit of the content of the thermoplastic elastomer in the sheet material is set appropriately, but is preferably 55% by mass or less, more preferably 50% by mass or less, and even more preferably 45% by mass or less. When the content is 55% by mass or less, the moldability can be further improved. In particular, the moldability in the vacuum molding process can be improved. In addition, when the sheet is taken off after vacuum molding or the edge of the sheet is cut, the workability can be improved. In particular, when the content of the thermoplastic elastomer in the sheet material is 45% by mass or less, the manufacturing characteristics can be improved in the sheet folding process described below.

[0025] With respect to the sheet material constituting the core layer 20 and the sheet materials constituting the skin layers 30, 40, it is preferable that the sheet material constituting the core layer 20 has a higher content of thermoplastic elastomer.

[0026] The lower limit of the bending elastic modulus of the sheet material is 200 MPa or more, preferably 250 MPa or more, and more preferably 300 PMa or more. By making the bending elastic modulus of the sheet material 200 MPa or more, the formability can be improved when obtaining a sheet material having a predetermined uneven shape formed by vacuum forming. In addition, when the bending elastic modulus is 300 PMa or more, the manufacturing characteristics can be improved in the sheet folding process.

[0027] The upper limit of the flexural modulus of the sheet material is not particularly limited, but is preferably 800 MPa or less, and more preferably 700 MPa or less. By setting the flexural modulus of the sheet material to 700 MPa or less, the restoring property against external pressure can be further improved.

[0028] The bending modulus of elasticity of the sheet material can be measured by a test method conforming to JIS K7171. (Hollow structure) 1(b) and 1(c), the core layer 20 is formed by folding a sheet material 100 obtained by forming a sheet made of a resin material including a polypropylene resin and a thermoplastic elastomer into a predetermined shape. The core layer 20 is composed of an upper wall portion 21, a lower wall portion 22, and a side wall portion 23 that is erected between the upper wall portion 21 and the lower wall portion 22 and forms a hexagonal cylindrical wall portion. The upper wall portion 21, the lower wall portion 22, and the side wall portion 23 define hexagonal columnar cells S inside the core layer 20.

[0029] The cells S formed inside the core layer 20 include a first cell S1 and a second cell S2, which have different structures. As shown in FIG. 1(b), the upper end of the first cell S1 is closed from the outside by two layers, the upper wall portion 21a and the upper wall portion 21b, and the lower end is closed by a single layer of the lower wall portion 22a. On the other hand, as shown in FIG. 1(c), the lower end of the second cell S2 is closed from the outside by two layers, the lower wall portion 22a and the lower wall portion 22b, and the upper end is closed by a single layer of the upper wall portion 21a. The upper wall portion 21a constituting the first cell S1 and the upper wall portion 21a constituting the second cell S2 are in communication with each other. The lower wall portion 22a constituting the first cell S1 and the lower wall portion 22a constituting the second cell S2 are in communication with each other.

[0030] 1(a), the first cells S1 and the second cells S2 are arranged in a row in the X direction, with the first cells S1 adjacent to each other or the second cells S2 adjacent to each other. In the Y direction perpendicular to the X direction, the rows of the first cells S1 and the rows of the second cells S2 are arranged alternately.

[0031] 1(a) to 1(c), adjacent first cells S1 and adjacent second cells S2 are separated by sidewalls 23 having a two-layer structure including a first sidewall 23a and a second sidewall 23b, which are formed perpendicular to the upper wall 21 and the lower wall 22. On the other hand, adjacent first cells S1 and second cells S2 are separated by sidewalls 23 having a single-layer structure formed perpendicular to the upper wall 21 and the lower wall 22.

[0032] 1(b) and 1(c), the first side wall portion 23a and the second side wall portion 23b are joined to each other at their vertical intermediate portions via a joint portion 23c. The first side wall portion 23a and the second side wall portion 23b are also bonded to each other by thermal welding at their upper and lower edges.

[0033] The joint 23c in this embodiment is formed of an adhesive layer, and the entire surface of the first side wall 23a and the entire surface of the second side wall 23b are joined (adhered) via the joint 23c (adhesive layer). Therefore, the upper and lower edges of the first side wall 23a and the second side wall 23b are joined via the joint 23c, and there are also parts where the thermoplastic resin is thermally melted and thermally welded. The material of the adhesive constituting the adhesive layer is a conventionally known one and is not particularly limited.

[0034] The skin layers 30, 40 are respectively bonded to the upper surface of the upper wall portion 21a and the lower surface of the lower wall portion 22a of the core layer 20. Specifically, the skin layer 30 is bonded to the upper surfaces of the upper wall portion 21a of the first cell S1 and the upper wall portion 21a of the second cell S2. Moreover, the skin layer 40 is bonded to the lower surfaces of the lower wall portion 22a of the first cell S1 and the lower wall portion 22a of the second cell S2.

[0035] Therefore, the upper surface of the hollow structure 10 in the first cell S1 has a three-layer structure consisting of the upper wall portions 21a and 21b of the core layer 20 and the skin layer 30, and the second cell S2 has a two-layer structure consisting of the upper wall portion 21a of the core layer 20 and the skin layer 30. The lower surface of the hollow structure 10 in the first cell S1 has a two-layer structure consisting of the lower wall portion 22a of the core layer 20 and the skin layer 40, and the second cell S2 has a three-layer structure consisting of the lower wall portions 22a and 22b of the core layer 20 and the skin layer 40.

[0036] (Manufacturing method) Next, a method for manufacturing the hollow structural body 10 of this embodiment will be described with reference to FIG. The manufacturing method of the hollow structure 10 includes a molding step, a joint forming step, a folding step, a joining step, and a skin layer joining step. A known manufacturing method can be applied to each of the steps. The molding step is a step of molding the sheet material 100 from a resin sheet containing polypropylene resin and a thermoplastic elastomer. The joint forming step is a step of applying an adhesive to the sheet material 100 to form an adhesive layer that constitutes the joint 23c. The folding step is a step of folding the sheet material 100 and arranging a plurality of cells S in parallel to form the core layer 20. The joining step is a step of joining the first side wall portion 23a and the second side wall portion 23b, which have a two-layer structure, via the joint 23c. The skin layer joining step is a step of joining the skin layers 30, 40 to both the upper and lower surfaces of the core layer 20 to form the hollow structure 10. Each of these steps for manufacturing the hollow structure 10 can be performed in a series of steps using a known device.

[0037] An example of a known apparatus has the following configuration (not shown): In the apparatus, from the upstream side, a sheet roll on which a resin sheet containing polypropylene resin and thermoplastic elastomer is wound, a vacuum forming drum for the molding process, a transport roll for the joint forming process, a first conveyor for the folding process, a second conveyor for the joining process, and a third conveyor for the skin layer joining process are arranged. Between the second conveyor for the joining process and the third conveyor for the skin layer joining process, a sheet roll on which a sheet serving as the raw material for the skin layers 30 and 40 is wound is arranged.

[0038] (molding process) In the molding process, a resin sheet containing polypropylene resin and a thermoplastic elastomer wound on a sheet roll is supplied to a vacuum forming drum, and a sheet material 100 is formed in which a predetermined uneven shape is transferred from the vacuum forming drum to the sheet. The vacuum forming drum is supported so as to be rotatable and heated to a predetermined temperature. A cylindrical molding die is attached to the outer periphery of the vacuum forming drum, and is configured so that a vacuum can be drawn through a through hole formed in the molding die. The outer periphery of the molding die is formed with an uneven shape similar to the uneven shape formed in the sheet material 100 (flat region 110 and bulging region 120 described below) so that the X direction of the sheet material 100 is aligned with the circumferential direction of the outer periphery of the molding die.

[0039] As shown in FIG. 2(a), the sheet material 100 has band-shaped planar regions 110 and bulging regions 120 arranged alternately in the longitudinal direction (X direction) of the sheet material 100. In the bulging region 120, a first bulging portion 121 having a cross section of a downward groove shape consisting of an upper surface and a pair of side surfaces is formed over the entire extending direction (Y direction) of the bulging region 120. Note that the angle between the upper surface and the side surface of the first bulging portion 121 is preferably 90°, and as a result, the cross section of the first bulging portion 121 has a downward U-shape. In addition, the width of the first bulging portion 121 (the length of the short side of the upper surface) is set to be equal to the width of the planar region 110 and twice the length of the bulging height of the first bulging portion 121 (the length of the short side of the side surface).

[0040] In addition, in the bulging region 120, a plurality of second bulging portions 122, each of which has a cross-sectional shape that is a trapezoid obtained by bisecting a regular hexagon at its longest diagonal, are formed so as to be perpendicular to the first bulging portion 121. The bulging height of the second bulging portion 122 is set to be equal to the bulging height of the first bulging portion 121. In addition, the interval between adjacent second bulging portions 122 is equal to the width of the upper surface of the second bulging portion 122.

[0041] In this manner, in the forming process, a vacuum forming method utilizing the plasticity of the sheet is used to obtain a sheet material 100 having a flat region 110 and a bulging region 120 formed by partially bulging the sheet upward or downward.

[0042] If necessary, unnecessary ends (edges) on both sides of the obtained sheet material 100 may be cut off. The cutting method may be a known method, such as a cutting method using a blade such as a cutter or scissors, or a method using heat.

[0043] (Joint forming process) In the joint forming process, the sheet material 100 is transported between a pair of transport rolls. A hot melt adhesive is sequentially supplied in a molten state to the surfaces of the transport rolls. As a result, the molten adhesive is applied to the surface of the sheet material that has passed between the pair of transport rolls. The adhesive is applied in a thin film form to the entire upper surface of the bulging region 120 on the upper surface of the sheet material 100, and is applied in a thin film form to the entire lower surface of the flat region 110 on the lower surface of the sheet material 100.

[0044] (Folding process) In the folding process, the sheet material 100 on which the adhesive is applied to form an adhesive layer is conveyed downstream by the first conveyor while its vertical movement is restricted. At this time, the conveying speed by the first conveyor is set to be slower than the rotation speed of the conveying rolls for the joint forming process arranged on the upstream side of the first conveyor. In other words, the conveying speed by the first conveyor is set to be slower than the supply speed of the sheet material 100 that has passed through the conveying rolls for the joint forming process. In addition, the first conveyor is provided with a heating device for heating the temperature between the first conveyors to a predetermined temperature. Therefore, when the sheet material 100 is conveyed between the first conveyors, it is folded while being compressed in the downstream direction while being heated, and the core layer 20 is formed.

[0045] As shown in Figures 2(a) and 2(b), in the folding process, the sheet material 100 configured as described above is compressed while being folded along fold lines P and Q due to the difference in feed speed with the conveying rolls in the joint formation process, thereby forming a core layer 20.

[0046] Specifically, the sheet material 100 is valley-folded at a folding line P between the flat region 110 and the bulging region 120, and mountain-folded at a folding line Q between the top and side of the first bulging portion 121, contracting in the X direction. Then, as shown in Figs. 2(b) and 2(c), the top and side of the first bulging portion 121 are folded over, and the end face of the second bulging portion 122 is folded over the flat region 110. As a result, one rectangular column-shaped partition 130 extending in the Y direction is formed for one bulging region 120. Such partitions 130 are continuously formed in the X direction to form the hollow plate-shaped core layer 20.

[0047] When the sheet material 100 is folded and formed as described above, the upper wall portion 21 having a two-layer structure is formed by the upper surface and side surface of the first bulging portion 121 and the upper wall portion 21a and the upper wall portion 21b of the core layer 20. The end surface of the second bulging portion 122 and the flat area 110 form the lower wall portion 22 having a two-layer structure and the lower wall portion 22b and the lower wall portion 22a of the core layer 20. As shown in FIG. 2(c), the portion of the upper wall portion 21 where the upper surface and side surface of the first bulging portion 121 are folded over to form the two-layer structure is the overlapping portion 131. The portion of the lower wall portion 22 where the end surface of the second bulging portion 122 and the flat area 110 are folded over to form the two-layer structure is the overlapping portion 131.

[0048] Moreover, the hexagonal columnar region formed by folding the second bulging portion 122 becomes the second cell S2 in the core layer 20, and the hexagonal columnar region formed between a pair of adjacent partitions 130 becomes the first cell S1 in the core layer 20. In this embodiment, the upper surface and side surface of the second bulging portion 122 form the side wall portion 23 of the second cell S2. The side surface of the second bulging portion 122 and the flat portion located between the second bulging portions 122 in the bulging region 120 form the side wall portion 23 of the first cell S1. The contact portion between the upper surfaces of the second bulging portions 122 and the contact portion between the flat portions in the bulging region 120 form the side wall portion 23 having a two-layer structure. In the first cell S1, the upper portion is partitioned by a pair of overlapping portions 131, and in the second cell S2, the lower portion is partitioned by a pair of overlapping portions 131.

[0049] In the joint forming process, the molten adhesive is applied by the transport roll, and in the folding process, the first conveyor is heated by a heating device. Therefore, when the folded core layer 20 passes through the first conveyor, the adhesive of the adhesive layer is in a molten state. In addition, the core layer 20 heated by the heating device of the first conveyor is pressed by the first conveyor. Therefore, the upper and lower edges of the two-layered side wall portion 23 are thermally welded.

[0050] (Joining process) As shown in Figures 2(b) and 2(c), the core layer 20 formed by the folding process moves toward the second conveyor. The folded core layer 20 passes through the second conveyor while maintaining its shape.

[0051] The second conveyor is not provided with a heating device. Therefore, the molten adhesive applied to the two-layered side wall portion 23 of the core layer 20 is cooled and solidified. As a result, the first side wall portion 23a and the second side wall portion 23b, which form the two-layered structure, are bonded via the bonding portion 23c formed by the cooling and solidification of the adhesive.

[0052] (Skin layer bonding process) Subsequently, the core layer 20 obtained by the bonding step moves toward the third conveyor. The third conveyor is also not provided with a heating device. Near the entrance of the third conveyor, sheet rolls are disposed, each of which is wound with a resin sheet containing polypropylene resin and thermoplastic elastomer, which are the raw materials of the skin layers 30 and 40. An adhesive is applied to the sheet wound on the sheet roll. The adhesive here is preferably a hot melt adhesive, similar to the adhesive layer constituting the bonding portion 23c.

[0053] By passing between the third conveyors, the sheets wound on the sheet roll are sequentially supplied to the upper surface of the upper wall portion 21a and the lower surface of the lower wall portion 22a of the core layer 20. In this state, the adhesive applied to the sheet from the sheet roll is in a molten state. On the other hand, the third conveyor is not provided with a heating device. Therefore, the sheets supplied to the upper surface of the upper wall portion 21a and the lower surface of the lower wall portion 22a of the core layer 20 are bonded by cooling and solidifying the applied adhesive. As a result, a hollow structure 10 is obtained in which the skin layers 30, 40 are bonded to the upper surface of the upper wall portion 21a and the lower surface of the lower wall portion 22a of the core layer 20 via the adhesive layer.

[0054] (action) The function of the hollow structure 10 will now be described. The hollow structure 10 of this embodiment is formed from a sheet material 100 made of a resin material containing polypropylene resin and a thermoplastic elastomer. Therefore, the hollow structure 10 of this embodiment has improved restoring ability against external pressure compared to a hollow structure made of a sheet material made of synthetic resin made of polypropylene resin. For example, in a compression strength test (ASTM C365), the hollow structure 10 of this embodiment has a lower compressive elastic modulus (MPa) and a higher maximum displacement (mm) compared to a hollow structure made of a sheet material made of synthetic resin made of polypropylene resin.

[0055] Moreover, in the hollow structure 10, the bending elastic modulus of the sheet material 100 is set to 200 MPa or more. Therefore, when a sheet material having a predetermined uneven shape is obtained by vacuum forming, the moldability can be improved. In particular, when the sheet material 100 is released from the vacuum forming drum during vacuum forming, the releasability of the sheet material 100 can be improved. Therefore, molding defects such as deformation, wrinkles, and tears of the sheet material 100 can be suppressed.

[0056] According to the hollow structural body 10 of this embodiment and the manufacturing method thereof, the following effects can be obtained. (1) When the content of thermoplastic elastomer in the sheet material 100 of the hollow structure 10 of this embodiment is 10% by mass or more, the hollow structure 10 can have improved resilience against external pressure compared to a hollow structure made of a sheet material made of a synthetic resin such as polypropylene resin.

[0057] (2) When the polypropylene resin, which is the raw material of the sheet material 100, is a block copolymer, the moldability can be further improved when obtaining the sheet material 100 having a predetermined uneven shape formed therein by vacuum forming. In particular, by improving the physical properties of the sheet material 100, the releasability of the sheet material 100 from the vacuum forming drum after vacuum forming can be improved.

[0058] (3) In the hollow structure 10 of this embodiment, the sheet material 100 having a concave-convex shape is folded in a folding process to arrange a plurality of cells S side by side. When the sheet material 100 has a flexural modulus of 300 MPa or more, the manufacturing characteristics can be improved when the sheet material 100 is folded in the folding process.

[0059] (4) The manufacturing method of the hollow structure 10 includes a molding process in which the sheet material 100 is formed into a predetermined uneven shape by vacuum molding. Since the sheet material 100 includes a resin material including polypropylene resin and thermoplastic elastomer, the moldability is not affected in the vacuum molding process. Therefore, a hollow structure having excellent restorability against external pressure can be efficiently manufactured.

[0060] (5) The manufacturing method of the hollow structure 10 includes a folding process in which the sheet material 100 is folded to arrange a plurality of cells side by side after the molding process. In particular, when the sheet material 100 having a flexural modulus of 300 MPa or more is used, the folding process does not affect the manufacturing characteristics. Therefore, a hollow structure having excellent restoring properties against external pressure can be efficiently manufactured.

[0061] The above embodiment can be modified as follows: The above embodiment and the following modified examples can be applied in combination with each other to the extent that there is no technical contradiction. The sheet material constituting the core layer 20 may have a configuration other than the above-mentioned sheet material 100. For example, the core layer 80 may be formed from a sheet material 200 as shown in FIG.

[0062] As shown in FIG. 3(a), the sheet material 200 has band-shaped first bulging portions 210 and second bulging portions 220 arranged alternately in the width direction (Y direction). The first bulging portions 210 are formed in a shape that protrudes upward, and the second bulging portions 220 are formed in a shape that protrudes downward. The first bulging portions 210 and the second bulging portions 220 are arranged alternately so as to extend in the X direction. The first bulging portions 210 when the sheet material 200 is viewed from above and the second bulging portions 220 when the sheet material 200 is viewed from below have the same shape and are formed at positions shifted by 1 / 2 pitch in the X direction.

[0063] The first bulging portion 210 is composed of an upper surface 210a, a pair of side surfaces 210b, and a pair of end surfaces 210c, and its Y-direction cross-sectional shape is a trapezoid obtained by bisecting a regular hexagon by its longest diagonal. The pair of end surfaces 210c are formed at the position of the folding line P shown in Fig. 3(a). The angle between the end surfaces 210c and the upper surface 210a is approximately 90°.

[0064] On the other hand, the second bulging portion 220 is composed of a lower surface 220a, a pair of side surfaces 220b, and a pair of end surfaces 220c, and its Y-direction cross-sectional shape is a trapezoid obtained by bisecting a regular hexagon by the longest diagonal. The pair of end surfaces 220c are formed at the position of the folding line Q shown in FIG. 3(a). The angle between the end surface 220c and the lower surface 220a is about 90°. The length in the X-direction of the second bulging portion 220, that is, the length between the pair of end surfaces 220c, is the same as the length in the X-direction of the first bulging portion 210, that is, the length between the pair of end surfaces 210c. The end surface 220c of the second bulging portion 220 is located at the center of the first bulging portion 210 in the X-direction. Note that the side surface 210b of the first bulging portion 210 and the side surface 220b of the second bulging portion 220 are separated for convenience of explanation, but have the same configuration.

[0065] As shown in FIG. 3(b) and FIG. 3(c), the sheet material 200 is folded sequentially along the folding lines P and Q to form the core layer 80. Specifically, as shown in FIG. 3(b), the sheet material 200 is mountain-folded along the folding line P and valley-folded along the folding line Q. As shown in FIG. 3(c), in one first bulging portion 210, the sheet material 200 is valley-folded at the folding line Q provided in the center portion in the X direction, and the upper surface 210a on the right side in the X direction and the upper surface 210a on the left side in the X direction abut in an upright state. In the folded first bulging portion 210, the upper surface 210a on the right side in the X direction and the upper surface 210a on the left side in the X direction abut in an upright state form the two-layered side wall portion 83 of the core layer 80, and the side surface 210b forms the one-layered side wall portion 83 of the core layer 80. At the upper end of the side wall portion 83, an upper wall portion 81 consisting of end faces 210c of adjacent first bulging portions 210 is formed.

[0066] In addition, one second bulging portion 220 is folded at a folding line P provided at the center in the X direction between adjacent folding lines Q, and the lower surface 220a on the right side in the X direction and the lower surface 220a on the left side in the X direction abut in an upright state. In the folded second bulging portion 220, the lower surface 220a on the right side in the X direction and the lower surface 220a on the left side in the X direction abut in an upright state to form a two-layered side wall portion 83 of the core layer 80, and the side surface 220b forms a one-layered side wall portion 83 of the core layer 80. At the lower end of the side wall portion 23, a lower wall portion 82 consisting of the end surfaces 220c of the adjacent second bulging portions 220 is formed. In addition, a hexagonal column-shaped region formed by folding the second bulging portion 220 becomes a second cell S2' in the core layer 80. The hexagonal columnar region formed by folding the first bulge portion 210 becomes the first cell S1' in the core layer 80. The second cell S2' has an opening at the upper end. The first cell S1' has an opening at the lower end. Therefore, it is preferable that the bending modulus of the sheet material 200 is higher than the bending modulus of the sheet material 100. This can further improve the manufacturing characteristics in the folding process in which the sheet material 200 is compressed. A hollow structure is formed by laminating and adhering skin layers (not shown) to the core layer 80 having openings in the upper and lower layers, respectively.

[0067] By blending a thermoplastic elastomer with polypropylene resin, the sheet material becomes more deformable, and therefore the welded area between the skin layer and the core layer 80 can be made larger than if the skin layer and the core layer 80 were made of polypropylene resin alone.

[0068] The core layer 20 of the hollow structure 10 does not have to be formed through a folding process in which the sheet material 100 is folded. A hollow structure that does not require a folding process may be used. As shown in FIG. 4(a), a hollow structure 50 may be employed in which a sheet material 300 having a concave and convex shape is formed by, for example, vacuum forming, and skin layers 330, 340 are laminated on the top and bottom of the sheet material 300, respectively.

[0069] As shown in FIG. 4(b) and FIG. 4(c), the sheet material 300 constituting the hollow structure 50 has a first bulge 310 formed in an approximately triangular pyramid shape protruding upward, and a second bulge 320 formed in an approximately triangular pyramid shape protruding downward. The hollow structure 50 has the first bulge 310 and the second bulge 320 formed by vacuum molding, so that the core layer 90 is formed without a folding process. The first bulge 310 when the sheet material 300 is viewed from above and the second bulge 320 when the sheet material 300 is viewed from below have the same shape. The first bulge 310 and the second bulge 320 are arranged in a staggered pattern in the X direction and the Y direction, respectively. The approximately triangular pyramid-shaped area partitioned and formed by the second bulge 320 becomes the second cell S4 in the core layer 90. The region having a substantially triangular pyramid shape defined by the first bulging portion 310 becomes the first cell S3 in the core layer 90.

[0070] The drop weight impact test (J) value of the hollow structures 10, 50 is set as appropriate, but is preferably set within the following range. Compared to a hollow structure of the same structure but made of a sheet material made of synthetic resin made of polypropylene resin, the drop weight impact test (J) value is preferably 1.2 to 3 times, more preferably 1.2 to 2 times. By specifying the value within such a range, the impact absorption characteristics of the hollow structure can be further improved.

[0071] The thicknesses of the sheets constituting the core layers 20, 80, 90 and the skin layers 30, 40, 330, 340 are set appropriately. From the viewpoint of bending strength, it is preferable that the sheet material constituting the skin layers is thicker than the sheet material constituting the core layers. From the viewpoint of compressive strength, it is preferable that the sheet material constituting the core layers is thicker than the sheet material constituting the skin layers.

[0072] The surface roughness of the sheets constituting the core layers 20, 80, 90 and the skin layers 30, 40, 330, 340 is set appropriately, but is preferably set within the following ranges. The lower limit of the arithmetic mean roughness (Ra) of the sheet constituting the core layer is preferably 0.15 μm or more, more preferably 0.4 μm or more. The upper limit of the arithmetic mean roughness (Ra) of the sheet constituting the core layer is preferably 0.5 μm or less.

[0073] The lower limit of the maximum height (Ry) of the sheet constituting the core layer is preferably 1 μm or more, more preferably 4 μm or more. The upper limit of the maximum height (Ry) of the sheet constituting the core layer is preferably 5 μm or less.

[0074] The lower limit of the ten-point average roughness (Rz) of the sheet constituting the core layer is preferably 0.5 μm or more, more preferably 2 μm or more. The upper limit of the ten-point average roughness (Rz) of the sheet constituting the core layer is preferably 3 μm or less.

[0075] When the surface roughness of the sheet constituting the core layer is equal to or greater than the lower limit, the contact area with the manufacturing equipment during the manufacturing process can be reduced, thereby reducing friction. This improves the formability of the sheet. When the surface roughness is equal to or less than the upper limit, the unevenness of the sheet can be reduced, thereby improving the formability, particularly in vacuum molding. Furthermore, the welding area can be increased during welding with the skin layer, thereby improving adhesion.

[0076] The lower limit of the arithmetic mean roughness (Ra) of the sheet constituting the skin layer is preferably 0.4 μm or more, more preferably 0.8 μm or more. The upper limit of the arithmetic mean roughness (Ra) of the sheet constituting the skin layer is preferably 2 μm or less, more preferably 1.5 μm or less.

[0077] The lower limit of the maximum height (Ry) of the sheet constituting the skin layer is preferably 3 μm or more, more preferably 5 μm or more. The upper limit of the maximum height (Ry) of the sheet constituting the skin layer is preferably 8 μm or less, more preferably 6 μm or less.

[0078] The lower limit of the ten-point average roughness (Rz) of the sheet constituting the skin layer is preferably 2 μm or more, more preferably 3 μm or more. The upper limit of the ten-point average roughness (Rz) of the sheet constituting the skin layer is preferably 7 μm or less, more preferably 5 μm or less.

[0079] When the surface roughness of the sheet constituting the skin layer is equal to or greater than the above lower limit, there is no need to perform a treatment such as increasing the surface roughness with a burner, etc., when performing printing, etc. On the other hand, when the surface roughness is equal to or less than the above upper limit, the unevenness of the sheet can be reduced, improving manufacturing characteristics.

[0080] In the hollow structure 10, the inner surface of the sheets constituting the skin layers 30, 40 is not rubbed against each other, and therefore has a lower surface roughness than the outer surface of the sheets. In addition, the surface roughness of the sheets constituting the core layer 20 is made higher than that of the inner surface of the skin layers 30, 40, thereby ensuring a welding area between the sheets. Therefore, the relationship between the surface roughness (a) of the outer surface of the sheets constituting the skin layers 30, 40, the surface roughness (b) of the inner surface of the sheets constituting the skin layers 30, 40, and the surface roughness (c) of the sheets constituting the core layer 20 is preferably a>c>b.

[0081] In the manufacturing method of the hollow structure 10 of this embodiment, the molding step, the joint forming step, the folding step, the joining step, and the skin layer joining step may be performed by a plurality of devices rather than one device. For example, the molding step of forming the sheet material 100 and the folding step may be performed by different devices, and the folding step and the skin layer joining step may be performed by different devices.

[0082] In the hollow structure 10 of the present embodiment, the cells S are partitioned and formed in a hexagonal column shape inside the core layer 20, but the shape of the cells S is not limited to this. For example, the cells S may be polygonal columns such as square columns or octagonal columns, or may be cylindrical.

[0083] The skin layers 30, 40 joined to the hollow structure 10 do not have to have a single-layer structure, and at least one of them may have a multi-layer structure. At least one of the skin layers 30, 40 may be omitted.

[0084] The resin material may further contain components that are typically used in resin materials, such as stabilizers and antioxidants for maintaining the quality of the sheet material, within the limits that do not impair the effects of the present invention. EXAMPLES

[0085] The hollow structure of this embodiment will be described in more detail with reference to the following examples. Note that the hollow structure is not limited to the configurations described in the examples. <Test Example 1: Formability test> Sheets were made from the resin materials shown in each example in Table 1 below, and hollow structures were manufactured to evaluate moldability. As the resin materials, a polystyrene-based elastomer with a compression set of 36% was used as the thermoplastic elastomer (TPE), and a block copolymer was used as the polypropylene resin (PP). They were mixed in the ratios shown in Examples 1 to 3 and Comparative Example 1 to make sheets.

[0086] The hollow structure shown in Figures 1 and 2 was manufactured by the method described in the above embodiment. The hollow structure had a total thickness of 13 mm, a core layer sheet thickness of 0.3 mm, and a skin layer sheet thickness of 0.6 mm. The core layer and skin layer were made of the same resin material.

[0087] The sheet materials used in each example were measured for flexural modulus and evaluated for manufacturing characteristics in the manufacturing process according to the following criteria. The results are shown in Table 1. (Flexural modulus of sheet material) Test pieces (80 x 10 x h4 mm) were prepared from the resin material of each example, and measurements were performed using the test method in accordance with JIS K7171.

[0088] (Vacuum forming / mold release) The moldability and ease of demolding in the vacuum molding drum during the vacuum molding process were evaluated.

[0089] ○: No problems were found in the molding and demolding processes, and the obtained sheet material had no deformation, wrinkles, or tears. ×: There is a problem in either the molding or demolding process, and the obtained sheet material is deformed, wrinkled, or torn. (Ear cut) After the vacuum forming process, the unnecessary edges (edges) on both sides of the sheet transported by the conveyor were cut off with a fixed cutter, and the ease of cutting was evaluated.

[0090] ○: If the ears were cut without any problems ×: Cutting failure or defects such as deformation or wrinkles in the sheet material (Seat folding process) The manufacturing characteristics in the sheet folding process were evaluated.

[0091] ○: If the sheet folds without any problems ×: Folding failure or failure such as crushing of the obtained hollow structure occurs

[0092] [Table 1] As shown in Table 1, good moldability was obtained in each of the Examples.

[0093] <Test Example 2: Resilience Test> Sheets were made from the resin materials shown in each example in Table 2 below, and hollow structures were manufactured and the restorability of the hollow structures was evaluated. The resin materials used were a polystyrene-based elastomer with a compression set of 36% as the thermoplastic elastomer (TPE), and a block copolymer as the polypropylene resin (PP). They were mixed in the ratio shown in each example to make sheets.

[0094] In Example 4 and Comparative Example 2, hollow structures shown in Figures 1 and 2 were manufactured. The hollow structures had a total thickness of 13 mm, a sheet thickness constituting the core layer of 0.3 mm, and a sheet thickness constituting the skin layer of 0.6 mm.

[0095] In Example 5 and Comparative Example 3, hollow structures shown in Fig. 4 were manufactured. The hollow structures had a total thickness of 5.2 mm, a sheet thickness constituting the core layer of 0.3 mm, and a sheet thickness constituting the skin layer of 0.6 mm.

[0096] Using the obtained hollow structure, a compressive strength test was conducted to measure the restorability. The compressive elastic modulus (MPa), maximum test force (N), and maximum displacement (mm) were measured using a test method based on ASTM C365. Note that the maximum displacement refers to the point at which the core buckles. If the value is less than the maximum displacement, restoration is possible. The results are shown in Table 2.

[0097] Furthermore, a drop weight impact test (J) was carried out on the hollow structures of Example 4 and Comparative Example 2. The drop weight impact test was carried out in accordance with the standard: JIS K7211-2, using a test machine: CEAST9350 model manufactured by Instron Co., Ltd. The results are shown in Table 2.

[0098] [Table 2] As shown in Table 2, by comparing Example 4 and Comparative Example 2, it is confirmed that the compressive modulus value is significantly reduced by using polypropylene and a thermoplastic elastomer in combination as the resin material. It is also confirmed that the maximum point displacement is significantly increased. In other words, it is confirmed that the hollow structure of Example 4 has significantly improved restoring ability against external pressure compared to the hollow structure of Comparative Example 2. As shown in Example 5 and Comparative Example 3, it is confirmed that the hollow structure of Example 5 has significantly improved restoring ability against external pressure compared to the hollow structure of Comparative Example 2, even when the configuration of the hollow structure is changed.

[0099] It was also confirmed that the hollow structure of Example 4 absorbed approximately 1.4 times as much energy as the hollow structure of Comparative Example 2 in a drop weight impact test. <Test Example 3: Surface Roughness Test> (1) A 0.3 mm thick sheet constituting the core layer was prepared from the resin material shown in each example in Table 3 below, and the surface roughness was measured. The surface roughness was measured in accordance with the standard: JIS B0601-1994, and the arithmetic mean roughness (Ra), maximum height (Ry), and ten-point mean roughness (Rz) were measured using a measuring instrument: Mitutoyo SJ-210. N=3 was measured, and the average values ​​are shown in Table 3.

[0100] [Table 3] As shown in Table 3, it was confirmed that the sheet of Example 6, which contained 30% thermoplastic elastomer (TPE) as the resin material, had a higher surface roughness than the sheet of Comparative Example 4, which was made of 100% polypropylene.

[0101] For example, a sheet made of thermoplastic elastomer (TPE) has a higher friction coefficient than a sheet made of polypropylene (PP). Therefore, the sheet is likely to adhere to the mold during vacuum molding, and is somewhat inferior in releasability. For the same reason, the sheet is likely to adhere to the conveying belt or sheet during transportation in the manufacturing process and during the folding process of the core layer. Therefore, it is preferable that the surface roughness of the sheet containing thermoplastic elastomer (TPE) constituting the core layer is higher than that of a sheet made of polypropylene (PP).

[0102] (2) A 0.6 mm thick sheet constituting the skin layer was prepared from the resin material shown in each example in Table 4 below, and the surface roughness was measured. The surface roughness was measured in accordance with the standard: JIS B0601-1994, and the arithmetic mean roughness (Ra), maximum height (Ry), and ten-point mean roughness (Rz) were measured using a measuring instrument: Mitutoyo SJ-210. N=3 measurements were made, and the average values ​​are shown in Table 4.

[0103] [Table 4] As shown in Table 4, it was confirmed that the sheet of Example 7 containing 40% thermoplastic elastomer (TPE) had a higher surface roughness than the sheet of Comparative Example 5 which contained 100% polypropylene as the resin material. [Explanation of symbols]

[0104] 10,50…Hollow structure 20, 80, 90...Core layer 30, 40, 330, 340…Skin layer 100, 200, 300...Sheet material, S: cell, S1, S3…First cell S2, S4...second cell.

Claims

1. A hollow structure having a core layer in which a plurality of cells are arranged side by side, the core layer being made of a sheet material having a predetermined uneven shape formed by vacuum molding, The sheet material is made of a resin material containing a polypropylene resin and a thermoplastic elastomer, and the bending modulus of the sheet material is 200 MPa or more.

2. The hollow structure according to claim 1 , wherein the content of the thermoplastic elastomer in the sheet material is 10% by mass or more.

3. The hollow structural body according to claim 1 , wherein the polypropylene resin is a block copolymer.

4. 2. The hollow structure according to claim 1, wherein the sheet material having the concave and convex shape is folded to form a plurality of cells arranged in parallel, and the bending elastic modulus of the sheet material is 300 MPa or more.

5. A method for producing a hollow structural body according to any one of claims 1 to 3, comprising the steps of: A method for manufacturing a hollow structure, comprising a molding step in which the sheet material is vacuum molded to form a predetermined uneven shape.

6. The method for manufacturing a hollow structure according to claim 5, further comprising a folding step of folding the sheet material after the molding step to arrange a plurality of cells side by side, the sheet material having a flexural modulus of elasticity of 300 MPa or more.

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